Most of the machines that are classified as presses are made to have a frame one part of which is called a platen and another part of which is moveable toward and away from the platen. The moveable part is often called a ram, and it is moved by a mechanism which serves to apply a force to the ram to which reaction is developed in the frame. The mechanism is interposed between the frame and ram, and it changes dimension in its operation to move the ram by an amount called the stroke.
The work to be done is accomplished by the force exerted on a tool or workpiece positioned between the ram and the platen as those elements are forced together, i.e. when the press is "closed."
In most press designs the length of the stroke is fixed and, while the separation between ram and platen in the "closed" condition may be adjustable, it is not variable within a given press stroke. Because of that, there is a danger to the press and to the workpiece if, for some reason, such, for example, as the introduction of additional workpieces between ram and press, the forces or physical dimensions in the system were to change. The danger is increased when the workpiece or material to be pressed is subjected to the action of tool elements carried by one or both of the platen and ram.
If something does go awry, if a tool or workpiece breaks, or if a workpiece is not removed before the succeeding stroke, it may be necessary to stop the ram or take some other remedial action. One solution is to make one element in the system, a tool element or the platen moveable in response to forces in excess of some predetermined force.
In some cases a floating element is provided. The element is carried by a spring cushion formed by an elastomeric material or, more often, actual steel springs.
In presses used to form injection molded plastic parts, the problem of protecting against excessive force application as the press is closed is compounded by the fact that the press is often mounted horizontally such that gravitational forces act in a direction transverse to the pressing force and because it must be structured to receive raw materials injected under high pressure. Thus, it is common in injection molding presses to employ two platens, one fixed and the other carried by the fixed one and moveable relative to it against the bias of steel springs that are interposed between them. The tools for such presses are made of separable sections one carried by the ram and the other by the floating platen. Change in the degree of separation of the floating and fixed platens is caused by application of closing force to the ram, and can be compared, for different degrees of platen separation, to the position of the elements in the mechanism that applies force to the ram. In practice, the comparison is made at one point in the cycle of ram movement. The immediate protective action must be to stop the ram.